Zeolites are widely used as solid acid catalysts for conversion of biomass and petroleum-based feedstocks to chemicals and fuels. While the predominant use of zeolites remains for Brønsted acid catalyzed reactions, the catalytic capabilities of zeolites have greatly expanded with the introduction of Lewis acidic zeolites, such as TS-1, CIT-6, Sn-BEA, and other Lewis acidic zeolites. These powerful Lewis acidic zeolites can catalyze a whole new landscape of intriguing chemical reactions such as Meerwein-Poondorf-Verley reductions (MPV), aldol condensations, and the isomerization of glucose to fructose. Existing Lewis acidic zeolites include isolated metal centers, such as Ti, Zn, Hf, Zr, and Sn. Intriguingly, enzymes that catalyze similar reactions often include pairs of metal centers as opposed to isolated metal centers. In these enzymes, the catalytic pairs of metal centers are thought to increase both selectivity and activity. However, paired metal centers are challenging to achieve in heterogeneous catalytic materials.
Translating these beneficial features of enzymes to heterogeneous catalytic materials has been a longstanding challenge for the field of catalysis. Creating catalysts that include paired sites has been a challenge because heterogeneous catalytic materials typically possess catalytic sites that are non-uniform and/or randomly distributed. Yet, enzymes and homogeneous catalysis clearly demonstrate a benefit to creating catalytic material with paired sites. Numerous examples from homogeneous catalyst demonstrate that catalytic pairs can influence activity and selectivity. The key challenge remains how to achieve these catalytic pairs in heterogeneous catalytic materials such as zeolites. Zeolites are attractive targets since they are crystalline, can be shape selective, and can be made hydrophobic to enable Lewis acid chemistry in water. However, zeolites are highly challenging because these materials are synthesized using crystallization procedures that are only beginning to be understood beyond a phenomenological level.